1. Statement of the Technical Field
The invention is directed to a communication system. In particular, the invention is directed to a system and method for applying Decision Feedback Equalization (DFE) techniques in the receiver of a communication system that employs spreading in the frequency domain of a communication signal modulated according to orthogonal frequency division multiplexing (OFDM).
2. Description of the Related Art
In OFDM communication systems, the frequencies and modulation of a frequency-division multiplexing (FDM) communication signal are arranged orthogonal with each other to eliminate interference between signals on each frequency. The low-rate modulation and relatively low symbol rate of an OFDM system compared to the channel time characteristics reduces the system's sensitivity to multipath propagation issues. In an OFDM system, a number of low symbol-rate data streams are transmitted on separate narrow band subcarriers using multiple frequencies simultaneously instead of transmitting a single, high symbol-rate stream on one wide frequency channel. These multiple subcarriers have the advantage that the channel propagation effects are generally more constant over a given sub-channel than over the entire channel as a whole. A signal modulated with conventional in-phase/quadrature (I/Q) modulation can be transmitted over the individual subcarriers.
An OFDM signal can be considered the sum of a number of orthogonal subcarrier signals, with baseband data being independently modulated on each individual subcarrier, for example, by Quadrature Amplitude Modulation (QAM) or Phase-Shift Keying (PSK). The subcarrier signals may be combined for transmission through the use of an N point Inverse Fast Fourier Transform algorithm (IFFT), where N is equal to the number of subcarriers. The output of the IFFT is typically modulated by a main Radio Frequency (RF) carrier. A receiver, after demodulating the received signal from the main RF carrier can reconstruct the subcarriers by performing a Fast Fourier Transform (FFT) on the received signal.
OFDM communication systems have high spectrum efficiency (a high number of bits per second per Hz of bandwidth), simple mitigation of multipath interference, and enhanced noise immunity compare to many other communication systems. However, OFDM communications systems suffer from carrier frequency offsets, timing drift, and non zero Doppler spread, which cause time variations in the phase of the channel. Additionally, frequency selective fading in the channel may cause transmission nulls within the OFDM signal's transmission spectrum.
The receiver that is to receive the OFDM signal requires a minimum signal-to-noise ratio (SNR) per subcarrier in order to demodulate and decode the signal with an acceptably low bit error rate (BER). If there is other unwanted energy within the transmission spectrum, the SNR can decrease causing an increase in BER. The unwanted energy can be noise from other sources. A conventional OFDM signal is susceptible to such interferers because of the required minimum SNR per subcarrier for an acceptably low BER. Further, frequency selective fading in the channel causes transmission nulls within the OFDM signal's transmission spectrum, which selectively reduces the SNR on certain subcarriers within those nulls, depending on their frequency location, leading to an undesirable increase in BER.
U.S. Patent Application Publication No. 2008/0043861, which is incorporated by reference herein, describes the use of orthogonal transforms to spread OFDM signals in the frequency domain in order to lessen the effects of narrow band interference. Spreading the OFDM signal in the frequency domain can reduce the effect of frequency selective fading by providing frequency-domain processing gain.
Generally, a wireless communication receiver requires an equalizer to account for the effects of the RF carrier signal propagating from a transmitter to the receiver. For example, effects due to multipath fading must be accounted for. Multipath fading can cause both amplitude and phase variation across the channel. Multipath fading is caused by objects in the environment that act as reflectors to create multiple paths that a transmitted signal traverses. At the receiver, multiple copies of the transmitted signal are received. Based on differences in the received phase, delay, and amplitude of each of the copies, the composite received signal will be attenuated differently and have a different phase at different frequencies. For example, at some frequencies, the multiple copies of the signal will constructively interfere such that the power received at the receiver is higher than it would be if only one of the copies was received. At other frequencies, the received copies destructively interfere such that the received signal is attenuated. If the transmitter, receiver, and all of the reflectors are completely at rest, the power received at a given frequency will typically remain constant. However, if the transmitter, receiver, or reflectors are moving, the received power will vary at a rate that is dependent on the transmission frequency and the relative velocity of the transmitter, receiver, and reflectors Additionally, timing offsets between the transmitter and the receiver, i.e., the relative error associated with the reference clocks included in the transmitter and the receiver, can also cause phase variation across the channel.
The equalizer structure generally used in OFDM systems places the channel equalization/tracking function within the de-spread domain. However, with frequency spread OFDM, this equalization solution only works for the case when the channel can be represented by a universal phase offset. Thus, this simple equalizer structure is inappropriate for more general cases involving timing drift and/or time dependent multipath.
Thus, there is a need for a generally applicable time dependent equalization solution for frequency domain spread OFDM.